Sexually transmitted diseases (STDs) are among the most common infectious diseases in the United States. Infectious diseases are defined as diseases that can be passed from one person to another.

STDs are normally spread through sexual activities, including vaginal intercourse, anal intercourse, oral sex (mouth-to-genital contact) or any other form of intimate contact involving genitals. The diseases can also be spread through sharing sexual devices that are not washed or covered with a condom. Many STDs can be passed from a mother to her infant before, during or after birth, during breastfeeding. Some STDs can even be passed through non-sexual contact with the skin of an infected person. In some cases, certain STDs (e.g., pubic lice, scabies) are also spread through contact with an infected person’s clothes, towels or linens. Although it is uncommon, some STDs (e.g., genital herpes) have also been spread through contact with toilet seats and hot tubs.

STDs can travel from person to person in bodily fluids, including semen, vaginal fluids and blood. Some can enter the body through tiny cuts or tears in the mouth, anus or genitals. STDs that are spread through blood, such as human immunodeficiency virus (HIV) and hepatitis, may also be transmitted through sharing needles used for injecting intravenous drugs. 

According to the U.S. Centers for Disease Control and Prevention (CDC), approximately 15.3 million new cases of STDs are reported each year in the United States. However, many STDs go undiagnosed and unreported. Therefore, the CDC estimates that a total of 19 million STD infections occur in the United States each year.

Anyone engaging in sexual activity is at risk of contracting an STD. This includes sexual activity between a man and woman or between people of the same gender. However, certain populations are more likely to acquire the diseases. According to the CDC, STDs disproportionately affect women, infants, young people and minorities. Of these groups, young people are particularly affected. Nearly half of all STD cases occur among young adults between the ages of 15 and 24, and nearly two-thirds of all STDs occur in people under the age of 25.

STDs are passed more often from men to women than the reverse because the exposed surface area is larger in women. In addition, the vagina acts like a reservoir that prolongs contact with infectious fluids. Microscopic injuries during intercourse are also more common in women than in men.

The high rate of STD infection may be the result of many factors, including young adults becoming sexually active at a younger age. In addition, young people are more likely to have multiple sexual partners and are more likely to have unprotected sex.

Complications of the diseases vary. Although STDs are often linked with unpleasant symptoms, including sores and foul smelling discharge, they can cause a variety of serious complications such as liver failure, certain cancers and neurological problems. Infection with certain diseases may also result in death, with HIV being the deadliest STD.

In general, complications caused by STDs tend to be more severe for women. A common complication of many STDs in women is pelvic inflammatory disease (PID), which can result in chronic pelvic pain, infertility and ectopic pregnancy. Ectopic pregnancy is a potentially fatal condition in which a fertilized egg develops outside the uterus.

Women with sexually transmitted diseases can infect their infants before, during or after birth. Some STDs like syphilis cross the placenta, infecting the fetus while it is in the uterus. Others, like herpes, gonorrhea and chlamydia, can be transmitted during delivery as the infantpasses through the birth canal (vagina). In addition, HIV can infect the infant during breastfeeding.

STDs also increase a person’s risk of acquiring the HIV virus. According to the CDC, people who are infected with STDs are at least two to five times more likely to acquire HIV when exposed to the virus during sexual contact. The risk of infection is increased because the action of the disease in the body places tissues at risk for further infection. In addition, people with HIV who are also infected with another STD are more likely to transmit the HIV virus through sexual contact.